The present invention relates to a method for bending pipes, rods, profiled sections and similar blanks.
According to a further aspect, the present invention relates to a device for bending pipes, rods, profiled sections and similar blanks.
The expression “method for bending pipes, rods, profiled sections and similar blanks” is to be intended as referred to the set of technological operations of plastic deformation of the blank in question, which are required to change the course of the axis thereof from a straight one to a curvilinear one according to a continuous or discontinuous path, by applying simple or composite mechanical stresses onto the blank and by properly constraining the blank itself. In the remaining part of the description, reference will be made for convenience's sake to the bending of pipes, although the invention is clearly applicable to the bending of any other similar blank, be it a bar, a profiled section etc.
The known bending methods differ from each other substantially in the way of applying the deformation forces or torques, and in the way of constraining the pipe, usually by means of bending tools (dies) suitably sized and shaped. The characteristic parameters of the bending method are the size (diameter and thickness) of the pipe, the material of the pipe and the spatial course of the axis of the pipe, which course is defined by the length of the straight portions between adjacent bends, by the bending radiuses and angles and by the relative spatial orientation of the bends. In particular, each bend of the final product of the bending method is defined by the bending radius, or centerline radius, and by the bending angle.
Nowadays, the most commonly used pipe bending methods are the draw bending, the stretch bending and the roll bending (or variable-radius bending).
The draw bending method is schematically illustrated in FIGS. 1A and 1B of the attached drawings and substantially consists in the following two steps:
a) the pipe to be bent, indicated 110, is clamped at its front end between a bending tool or die 112, which is able to rotate around an axis Z perpendicular to the axis X of the pipe 110, and a front clamping block 114 and is guided upstream of the front block 114 by a rear abutment shoe 116 which is usually mounted on a movable slider (not shown) so as to be able to slide along the direction of the axis X of the pipe 110 (hereinafter simply referred to as axial direction) to accompany the axial forward movement of the pipe itself (FIG. 1A); and
b) the die 112 is caused to rotate about the axis of rotation Z so as to draw the pipe 110 forwards while winding it around a shaped groove 118 of the die itself which extends along a curve of radius R, while the rear show 116 accompanies the axial forward movement of the pipe 110 and applies on it a reaction force perpendicular to the axial direction X, thereby producing on the pipe 110 a bend having a centerline radius substantially corresponding to the centerline radius R of the groove 118 of the die 112 (FIG. 1B).
The draw bending method is at the moment the most common one and is capable of offering the best results in terms of quality. In particular, this method makes it possible to obtain small centerline radiuses which are small, even smaller than once the diameter of the pipe, and of good quality. On the other hand, it has several limits, such as the fact that it requires to change the die when bends of different centerline radiuses have to be obtained or pipes of different diameters have to be worked, as well as the fact that it requires to use particularly complicated apparatuses to produce a sequence of bends with straight portions of extremely small or even null length interposed therebetween.
The stretch bending method is schematically illustrated in FIGS. 2A and 2B of the attached drawings, where parts and elements identical or corresponding to those of FIGS. 1A and 1B have been given the same reference numerals, and substantially consists in the following two steps:
a) the pipe 110 to be bent is clamped at its rear end by means of rear clamping blocks 114 so as to project forwards with respect to a stationary die 112 having a shaped groove 118 extending along a curvilinear path of centerline radius R, the pipe 110 being pressed against the groove by means of a bending shoe 116 capable of rotating around an axis of rotation Z which is perpendicular to the axis X of the pipe 110 and passes through the centre of curvature of the groove 118 (FIG. 2A); and
b) the bending shoe 116 is caused to rotate around the axis of rotation Z, thereby winding the pipe 110 onto the die 112 and producing on the pipe itself a bend having a centerline radius substantially corresponding to the centerline radius R of the groove 118 of the die 112 (FIG. 2B).
Therefore, the two known bending methods described above suffer both from the shortcoming of making it possible to obtain only bends of fixed centerline radius, that is, a centerline radius corresponding to that of the shaped groove of the die. In order to obtain bends with a different centerline radius, it is therefore necessary to change die and accordingly to stop the process. Accordingly, when the pipe must have a complex path with a plurality of bends of different centerline radiuses, a plurality of die changes, and hence a corresponding plurality of stops of the process, are necessary, which results in a significant increase in the duration of the work cycle. This results in a higher cost of the process, and hence of the final product. Moreover, in order to make it possible to change automatically tools having different centerline radiuses to reduce the duration of the tool-change downtimes, the machines have to be provided with special handling devices and are thus more complicated and expensive.
The roll bending method, or variable-radius bending method, is schematically illustrated in FIGS. 3A to 3C of the attached drawings, where parts and elements identical or corresponding to those of the preceding figures have been given the same reference numerals, and substantially consists in the following steps:
a) the pipe 110 to be bent is clamped at its rear end by a chuck 114 mounted on a chuck-carrying slider (not shown) which can slide in the direction X of the axis of the pipe 110 (FIG. 3A);
b) the pipe 110 is urged forwards by the chuck 114 through a stationary roller 112 acting as a die, which has a shaped groove 118 and is mounted so as to be able to rotate freely around an axis of rotation Z perpendicular to the axis X of the pipe 110, and a bending roller 116, mounted so as to be able to rotate freely around an axis of rotation Z′ perpendicular to the axis X of the pipe 110 and to rotate around the axis of rotation Z of the stationary roller 112 from a neutral position (illustrated in dashed line in FIG. 3A), in which the pipe 110 is not deformed, to a working position rotated with respect to the neutral position by an angle of rotation α which varies depending on the bending centerline radius of the bend to be obtained (illustrated in continuous line in FIG. 3A), in which position the pipe 110 is bent to the desired radius, the pipe 110 being also pressed by abutment rollers 120 which exert on the pipe a reaction force perpendicular to the axial direction X.
The bend thus obtained may comprise the following three zones depending on the desired result and on the bend immediately preceding or following the one in question:                a leading zone 110′ which is obtained during the movement (rotation) of the bending roller 116 from the neutral position to the working position while the pipe 110 is urged forwards by the chuck 114 (FIG. 3A);        an intermediate zone 110″ which has the desired centerline radius and is obtained by keeping the bending roller 116 still in the working position and causing the pipe 110 to move forwards by means of the chuck 114 (FIG. 3B); and        a trailing zone 110″′ which is obtained during the movement (rotation) of the bending roller 116 from the working position to the neutral position while the pipe 110 continues to be urged forwards by the chuck 114 (FIG. 3C).        
The chuck 114 may also be provided with a rotational movement around the axis X of the pipe 110 in order to obtain 3-D bends, in particular bends with a spiral course.
The roll bending method offers the advantage of making it possible to obtain bends with different centerline radiuses without having to stop the process to change die. On the other hand, it also has some limits, such as for example the fact that the length of the straight portions between two adjacent bends cannot be bought to zero, the fact that the results (in terms of final centerline radius of the pipe) cannot be perfectly repeated with varying mechanical characteristics of the material of the pipe under working, the difficulty of foreseeing the results (in terms of final centerline radius of the pipe) depending on the geometry, setting and movement of the bending apparatus, the fact that bends having a bending centerline radius about five times shorter than the diameter of the pipe under working cannot be obtained, and the fact that bends with constant radius from the start to the end cannot be obtained, since the use of the bending roller requires that the start (leading zone) and the end (trailing zone) of the bend have a fillet radius different from the desired bending centerline radius of the bend.
U.S. Pat. No. 5,111,675 discloses a variable-radius bending method in which the pipe is caused to move forwards first through a guide cylinder and then through a die having a bending tool in the form of a sleeve, which is supported so as to be able to swivel around an axis perpendicular to the axis of the pipe. The die is movable along a first direction parallel to the axis of the pipe to change the distance between the guide cylinder and the bending tool, and along a second direction perpendicular to the axis of the pipe to change the distance between the axis of the pipe and the centre of the bending tool. The movement of the die along these two directions makes it possible to adjust the bending centerline radius of the bend produced onto the pipe.
The above-mentioned U.S. Patent further discloses a device for carrying out the variable-radius bending of pipes according to the method briefly discussed above. Such a device suffers however from the shortcoming that it is not able to carry out the bending according to at least two different methods, for example the variable-radius bending method and the draw bending method. Moreover, the sleeve acting as a bending tool must be calibrated on the diameter of the pipe to be worked. A further shortcoming linked to the use of such a device is represented by the fact that the fillet radius between two consecutive bends cannot be eliminated.